AFP/ Beirut

Syrian government aircraft launched at least 15 strikes in and around the ancient city of Palymra on Monday, following its fall to the Islamic State group, a monitoring group said.

The Syrian Observatory for Human Rights said the raids were the most intense since the jihadists overran the city on Thursday.

"Since this morning, government aircraft have carried out at least 15 air strikes in Palmyra and the areas around it," Observatory director Rami Abdel Rahman said.

He said there were reports of casualties but he had no firm toll.

The strikes targeted several areas of the city, including some close to the city's famed Greco-Roman ruins, a UNESCO world heritage site, he said.

IS is accused of executing hundreds of people in and around Palmyra since it swept into the oasis city last week after a lightning advance across the desert from its stronghold in the Euphrates Valley to the east.

The Observatory said on Sunday that it had documented the executions of at least 217 people, among them 67 civilians, including 14 children.

Some of those killed had been beheaded, Abdel Rahman said, adding that the jihadists had also taken some 600 people prisoner.

Syrian state media said at least 400 civilians had been killed by IS in Palmyra, most of them women, children and old men.

The pro-government Al-Watan daily reported on Monday that the number of executions had risen to 450.